Tachycardia refers generally to health ailments wherein one or more chambers of the patient's heart are contracting at an abnormally high rate. Fibrillation refers to a particularly dangerous tachycardia condition wherein one or more chambers are contracting in a rapid chaotic manner such that effective pumping from the affected chamber effectively ceases or is at least markedly reduced. Atrial fibrillation, while reducing overall heart efficiency by reducing the effective filling of the ventricles and presenting an elevated risk of thrombosis in certain patients, is generally not an immediately life-threatening condition. Ventricular fibrillation, however, is immediately life-threatening due to the effective cessation of pumping blood from the heart and, if not rapidly interrupted and replaced with at least limited pumping effectiveness, is fatal.
Accordingly, implantable cardioverter defibrillators (ICDs) have been developed to provide ongoing monitoring and therapy for treatment of potential fibrillation conditions. ICDs generally function by automatically monitoring the patient's cardiac activity for possible onset of a fibrillation condition and, upon detection of such a condition, automatically generate and deliver a therapeutic stimulation configured to interrupt the heart's fibrillation in an attempt to restore effective contractions. The stimulation delivered to defibrillate a person's heart is a relatively high energy electrical shock (up to the order of tens of joules) delivered between implanted electrodes, at least one of which is typically in direct contact with the patient's cardiac tissue. Defibrillation waveforms are typically either monophasic, having a single polarity, or biphasic, having both positive and negative polarities. The defibrillation shock is typically realized by accumulating a charge in a relatively large capacitor drawing electrical energy from a battery of the ICD. A common waveform delivered to the patient is a well-known decaying waveform following the exponential discharge decay of the capacitor and which is frequently gated or truncated after partial discharge of the charged capacitor. The waveforms are clipped or truncated in this manner to reduce the likelihood of retriggering an arrhythmia with a tailing discharge of the capacitor.
Several design and patient care considerations present themselves in the implementation of implantable cardioverter defibrillators. First, as previously noted, the energy typically required to defibrillate a patient is relatively large, e.g., on the order of tens of joules. As the implantable devices depend on a battery for their operating power, including generation and delivery of the defibrillation shocks, the energy draw to charge and deliver a defibrillation shock is a significant design consideration in implementing an ICD. The ICDs are desirably as small as possible to reduce discomfort and inconvenience to the implantee, however, reducing the size of the implantable device correspondingly reduces the volume available for the battery, as well as other components of the implantable device.
Secondly, the relatively high energy shock delivered to defibrillate the patient can be extremely painful and traumatic in many applications. The ICD automatically monitors the patient for indications of fibrillation and can frequently determine a fibrillation condition and prepare and deliver a therapeutic defibrillation shock pre-syncope, e.g., while the patient is still conscious. Such shocks delivered to a conscious patient can be extremely painful and anxiety and anticipation of aperiodic delivery of painful stimulation can contribute to development of psychological trauma in many patients.
Thus, it will be understood that there is a strong desire to effectively defibrillate a patient with a lower energy and/or voltage shock both to reduce the energy draw on the battery, thereby facilitating use of smaller batteries, as well as to extend the useful life between elective battery replacement explantation and implantation procedures. Reducing the voltage of the defibrillation shock also reduces the pain sensation and psychological trauma experienced by the recipient.
In response to these goals, a variety of alternative defibrillation shock waveform generators have been developed to provide alternatives to the truncated exponential decay of a simple capacitive discharge. For example, U.S. Publication 2001/0031991 to Russial teaches a circuit for producing a defibrillation waveform having an arrangement of a relatively complicated controlled voltage source added in the return path of the patient as connected to the charge capacitor to control the current delivered to the patient. U.S. Pat. No. 6,208,896 to Mulhauser teaches an apparatus for providing defibrillation waveforms including step-up and step-down converters, but which delivers a relatively jagged and inefficient defibrillation shock to the patient. As previously noted, arrangements to provide advantageous alternative defibrillation shocks must also take into consideration the strong design goals of maintaining a desirably compact implantable device to limit discomfort and inconvenience to the implantee, thereby limiting the use of relatively bulky components, such as inductors, within the devices.
Thus it will be appreciated that there is a desire for an ICD system which can deliver defibrillation shocks more efficiently and less painfully to a patient while not significantly expanding the physical envelope and weight of the device. It would be desirable to provide such improved ICD systems while avoiding significant additional circuit and control system complexity. It would be particularly desirable for a system to enhance the waveform efficiency and/or reduce the pain stimulus of existing alternative waveform generators, such as via a supplement or retrofit.